Lithographic printing processes rely on the differential wetting characteristics of hydrophobic and hydrophilic surfaces. In practice an aluminum surface is roughened, anodised, conditioned and then coated with a light sensitive coating. Positive and negative images are normally formed on the surface of the printing plate by photographic methods. Development of the image results in removal of the organic coating from either the exposed or unexposed areas. The organic areas are oleophilic and will accept oil-based inks but will not water wet. In contrast the conditioned anodic oxide has a high surface energy and can accept either water or ink, however, when wet it will not accept ink. The roughening stage is critical for print quality and requires uniform topographies with surface features in the range 0.01-4 .mu.m. The actual range employed for any particular plate depends predominantly on the quality of paper and required print finish.
The most common method of achieving the high standard of roughening required for lithographic plates is to electrochemically treat the surface. However, the process has several limitations. Specifically it can only be run at slow speeds and it requires very high quantities of electrical power and the use of specialist materials. Production of these materials demands special and costly practices in order to ensure the high quality of the final product. Also, expensive waste treatment plant is required to treat the waste chemicals from anodising and graining aluminum. The present invention will increase line-speeds, require less power usage, will eliminate costly production methods, increase the range of alloys that can be used to make lithographic printing plates, and permit the use of other metallic and non-metallic substrates. Also, the treatment is environmentally good as it does not give rise to significant waste disposal problems.
It is known to prepare a lithographic printing plate by applying to a substrate a suspension or sol of preformed particles, and removing the liquid to leave a coating comprising the particles. The particles may be bound together by means of a polymer or by partial sintering, but organic polymers may affect the hydrophilic lipophilic balance of the surface while partial sintering may require heating to such high temperatures as to damage the substrate. Particulate surface coatings of this kind are described in U.S. Pat. No. 4,293,625; 4,330,605; 4,445,998; 4,456,670; 4,457,971; 4,420,549; 4,542,089; and 4,687,729.
U.S. Pat. No. 3,231,376 describes a lithographic plate coated with a Type B sol derived from a titanium or zirconium alkoxide.
U.S. Pat. No. 3,419,406 describes a lithographic plate comprising a grained aluminum substrate coated with a sol derived from an alkyl titanate. The grained substrate is responsible for the surface topography.
U.S. Pat. No. 4,522,912 describes a lithographic plate in which a metal substrate carries an electrodeposited chromium layer of rough crag-like character with recesses covered by glass-like films derived from an ammonium zirconate carbonate solution.